Method and system for maintaining a desired air flow through a fume hood

ABSTRACT

The present invention pertains to a system for controlling a fume hood with a face. The system comprises a device for controlling air flow through the fume hood based on air flow face velocity. The controlling device has a device for measuring pressure at the face of the fume hood which produces a signal corresponding to the pressure. Additionally, the system comprises a controller. The controller is remote from the fume hood controlling device but in communication with the controlling device. The present invention also pertains to a system for controlling a fume hood having a face. The system comprises a pitot tube assembly for determining a desired face velocity. The pitot tube produces a pitot signal corresponding to the face velocity. The system comprises a device for converting the pitot signal into a face velocity signal. The converting device is connected to the pitot tube. Additionally, the system comprises a processor for controlling air flow through the fume hood based on the face velocity. The processor is connected to the converting device. The present invention also pertains to a method for controlling air flow through a fume hood having a face. The method comprises the steps of sensing velocity of air at the face of the fume hood with a pitot tube assembly. Then there is the step of controlling air flow through the fume hood based on the velocity of air at the face of the fume hood.

This application is a continuation application of Ser. No. 08/729,941,now U.S. Pat. No. 5,946,221, filed Oct. 15, 1996, which is acontinuation application of Ser. No. 08/301,572 filed Sep. 7, 1994, nowabandoned.

FIELD OF THE INVENTION

The present invention is related to controllers. More specifically, thepresent invention is related to a method, system and an apparatus forfume hood controllers. The controllers can be networkable, orqualitatively tuned or based on face velocity and sash position of thefume hood.

BACKGROUND OF THE INVENTION

Laboratories wherein dangerous experiments or processes are performedrequire protection for the workers and the experiments in thelaboratory. One very common protection found in laboratories are fumehoods in which chemical reactions are conducted. The fume hoods have airdrawn out of them thus essentially preventing any toxic fumes fromescaping the fume hood into the laboratory and threatening theoperators. The velocity of air drawn through the fume hood sash iscontrolled to a value high enough to maintain safety for the operatorand low enough to provide non-turbulent air for the experiment ofprocess.

An additional protection that can be provided is to maintain the staticpressure in the laboratory at a lower or higher pressure than thepressure in the surrounding corridors of the building. A lower pressurewould prevent contaminants from exiting the laboratory in the case of anaccident.

A higher pressure would prevent contaminants from entering thelaboratory, as is the case in a clean room. Also, control of thelaboratory climate is required both for operator comfort and for certainexperiments or processes where strict temperature and humidity controlare necessary.

Moreover, in regard to the tuning of a processor, for instance, for fumehoods, there exists automatically tunable processors which respond tothe system into which they are integrated. But after the processor isinitially tuned to its system, to tune it in terms of how a user wouldlike the processor to respond with respect to the system, the user mustbe fluent in operation of the operating system of the processor in orderto retune it.

There are many schemes and apparatuses that provide such control andprotection to laboratories. However, heretofore, there have been nosystems that provide for integrated direct digital control oflaboratories. Additionally, there have been no systems that areautomatically tunable based on simple manual inputs to the system.

Furthermore, heretofore, fume hoods typically have utilized the sashposition as an integral part of the determination for face velocity orflow. The present invention eliminates the need to know sash position todetermine face velocity or flow.

SUMMARY OF THE INVENTION

The present invention pertains to a system for controlling a fume hoodwith a face. The system comprises means for controlling air flow throughthe fume hood based on air flow face velocity. The controlling means hasmeans for measuring pressure at the face of the fume hood which producesa signal corresponding to the pressure. Additionally, the systemcomprises a controller. The controller is remote from the fume hoodcontrolling means but in communication with the controlling means.

The present invention also pertains to a system for controlling a fumehood having a face. The system comprises a pitot tube assembly fordetermining a desired face velocity. The pitot tube produces a pitotsignal corresponding to the face velocity. The system comprises meansfor converting the pitot signal into a face velocity signal. Theconverting means is connected to the pitot tube. Additionally, thesystem comprises a processor for controlling air flow through the fumehood based on the face velocity. The processor is connected to theconverting means.

The present invention also pertains to a method for controlling air flowthrough a fume hood having a face. The method comprises the steps ofsensing velocity of air at the face of the fume hood with a pitot tubeassembly. Then there is the step of controlling air flow through thefume hood based on the velocity of air at the face of the fume hood.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, the preferred embodiment of the inventionand preferred methods of practicing the invention are illustrated inwhich:

FIG. 1 is a schematic representation of a system for controllinglaboratories having fume hoods.

FIG. 2 is a schematic representation of a laboratory.

FIG. 3 is a schematic representation of a microprocessor.

FIG. 4 is a schematic representation of the network configuration.

FIG. 5 is a schematic representation of a module.

FIG. 6 is a schematic representation of the laboratory control circuit

FIG. 7 is a schematic representation of a hood control circuit.

FIG. 8 is a key, with respect to elements of FIGS. 6 and 7, definingthem.

FIG. 9 is a schematic representation of another embodiment of a hoodcontrol circuit.

FIG. 10 is a block diagram of a qualitative tuning system.

FIG. 11 is a schematic representation of a keypad.

FIG. 12 a graph of a turbulence input membership function.

FIG. 13 is a graph of a response time input membership function.

FIG. 14 is a graph of a sensitivity input membership function.

FIG. 15 is a graph of a proportional output membership function.

FIG. 16 is a graph of an integral output membership function.

FIG. 17 is a graph of a derivative output membership function.

FIG. 18 is a schematic representation of a network with a controller anda system of the present invention in conjunction with fume hoods

FIG. 19 is a schematic representation of another embodiment of a hoodcontrol circuit.

FIG. 20 is a schematic representation of a fume hood control circuitwhich does not utilize sash position.

FIG. 21 is a graph of input linearization.

FIG. 22 is a graph of a characterization in regards to a damper.

FIG. 23 is a schematic representation of a damper with actuator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like reference numerals refer tosimilar or identical parts throughout the several views, and morespecifically to FIG. 1 thereof, there is shown a schematicrepresentation of a system 10 for controlling laboratories 12 havingfume hoods 14. The system 10 is comprised of a network 16 along whichinformation is carried. Preferably, the network 16 operates in thehalf-duplex mode, for instance, with a two-wire RS485 network.

The system 10 is also comprised of a controller 18 in contact with thenetwork 16 for receiving information from and providing information tothe network 16. Preferably, the controller 18 includes means 24 forrequesting information from each microprocessor 22 about its condition.Preferably, the requesting means 24 requests information from eachmicroprocessor 22 one at a time.

Additionally, the system 10 is comprised of means 20 for sensing alaboratory's 12 state. The state of the laboratory is defined as, atleast, the static pressure and the supply/exhaust differential of thelaboratory 22, and also the face velocity of the fume hood 14 in thelaboratory. The sensing means 20 is disposed in each laboratory 12. Thesensing means 20 preferably includes a static pressure sensor 26, asupply/exhaust differential sensor 28 and a face velocity sensor 30disposed in each laboratory 12, as shown in FIG. 2. FIG. 2 is aschematic representation of the laboratory 12. There can also beincluded a temperature sensor 32.

There is also a microprocessor 22 disposed in each laboratory 12 forreceiving information concerning the laboratory 12 from the respectivesensing means 20 and the controller 18 in order to maintain thelaboratory 12 at a predetermined state, and to provide information aboutthe laboratory 12 to the controller 18. Preferably, the controller 18and the microprocessors 22 maintain the respective laboratory 12 in thepredetermined state by maintaining their respective static pressure andsupply/exhaust differential as well as the face velocity of the hoods 14of a given laboratory 12.

The controller 18 and microprocessors 22 preferably operate in a masterslave relationship with the controller 18 being the master and themicroprocessor 22 being the slave. The master initiates allcommunications by sending messages. Messages are composed, for instance,of data bytes transmitted serially using standard asynchronous dataframes. These data frames can consist of one start bit, eight data bits,no parity bit, and one stop bit.

The master and each slave share the same network 16 for transmitting andreceiving (half-duplex). The master and each slave must be able toenable/disable their transmitters (not shown), so as not to interferewith other slaves' transmissions. The transmitter enable/disable shouldbe controlled such that the carrier enable is switched off concurrentlywith the end of the final stop bit of any transmission. The masterarbitrates when a given slave may respond with the simple rule that aslave only transmits in response to a message uniquely directed to it.See PUP guidelines (A document entitled “PUP Protocol Guidelines” isavailable from American Auto-Matrix, Inc. Please contact the PUPProtocol Committee and request Version 6.) for an example of a protocolthat can be used in the system.

In the operation of the preferred embodiment, a microprocessor 22 isdisposed in each laboratory 12, as shown in FIG. 3. FIG. 3 is aschematic representation of the microprocessor 22. The microprocessor 22is comprised of a fuse 32 which protects the microprocessor 22 fromelectrical overload. There is a transformer 34 for converting currentand voltage provided to the microprocessor 22 through the power inputport 36. There is a first switch block 38 with eight switches used toselect thermistor support for eight analog inputs. When any given switchis on (moved to the right), it operates as a thermistor and when anygiven switch is off (moved to the left), the switch operates in a normalmode, as is well known in the art.

There is a second switch block 40 with eight switches used to select acurrent or voltage mode for analog inputs one through eight. When theswitch is off, the voltage mode is utilized and when the switch is on,the current mode is utilized. There is an analog input port 42 whichreceives analog input wiring, i.e., high resolution (12-bit) inputdevices such as flow sensors, velocity sensors, the static pressuresensor, and the discharge air temperature sensor are hardwired(connected) to the analog input port 42 (TB3). A processor 44 processesthe information received by the microprocessor from the network 16 andsensors in the laboratory 12 and also provides information concerningthe respective laboratory 12 to the network 16. A universal input wiringinput port 46 receives analog/digital input wiring, i.e., low resolution(8-bit) and digital input devices such as the room temperature sensor,sash position sensors, the humidity sensor, and the emergency contactsare hardwired to input port 46 (TB1). A third switch block 48 with eightswitches is used to select thermistor support for the universal inputport 46 inputs one through eight.

The memory for the microprocessor 22 includes an executive eprom 50, anonvolatile ram 52, an application eprom 54 and an expansion eprom/ram56. The executive eprom 50 contains the basic operating routines of themicroprocessor. Input/output, communications, diagnostics, andinitialization routines as well as the utility routines for theapplication which include but are not limited to the math functions andthe PID control routines.

The non-volatile ram 52 is used for work space for the executive and theapplication as well as for storage of attributes and control parameters.

The application eprom 54 contains the laboratory/fume hood controlalgorithms.

The expansion eprom/ram 56 is used for extra application algorithmstorage or for extra non-volatile ram storage. In the present SOLO/FXconfiguration, this site is unused.

There is a fourth switch block 58 with eight switches that can be usedas determined for a given situation. The digital (binary) output port isused to connect devices to the microprocessor for annunciating alarmconditions and for general purpose digital outputs. A fifth switch block62 with eight switches is used to select a current or a voltage mode foranalog outputs five through eight of analog output port 66. There is athree-volt lithium smart battery 64 used to maintain the data in the ram52 in the event of a power failure. There is a sixth switch block 56that has eight switches that are used for analog outputs one throughfour of analog output port 66 to determine whether they should be in acurrent or voltage mode.

A first led 70 indicates whether the application eprom 54 is installedin the microprocessor 22. If the led 70 is off, it indicates that theapplication eprom 54 is installed in the microprocessor 22. A second led72, when flashing, indicates that the microprocessor 22 is operatingproperly. A third led 74, when on, indicates that the microprocessor 22is transmitting data to the controller 18. A fourth led 76 indicateswhen the microprocessor 22 is receiving data.

There is a jumper block 78 with two pins. When the jumper block 78 isinstalled, a termination resister is positioned for the RS485 network16. The microprocessor 22 connects to the network 16 through the networkconnection port 80.

The controller 18, as shown in FIG. 4 which is a schematicrepresentation of the network configuration, includes a module 82. FIG.5 shows a module 82 and is a schematic representation of the module 82.The module 82 includes a module processor 84 for providing the properinstructions to the various laboratory 12 microprocessor 22 via thenetwork 16, as well as for receiving information to better maintain theoverall system 10 from the various microprocessors 22 and thelaboratories 12. The module 82 also includes a reset button 86, a systemtask led 88 and a communications led 89. The reset button 86 is used toreset the communication module 82 without resetting the entire STARcontroller 18. The system task LED 88 is lit to indicate that thecommunication module 82 task is currently being serviced. Thecommunication LED 89 is lit when the communication module 82 is idle,i.e. not transmitting. There are also ports 90 which connect to themodule 82 to the dual RS45 communications network 16. In general, forthe preferred embodiment, the inputs and outputs are the following:

Analog Inputs AI1 Space Static Pressure AI2 Supply Air Flow AI3 ExhaustAir Flow AI4 Hood A Air Flow AI5 Hood B Air Flow AI6 Hood A FaceVelocity AI7 Hood B Face Velocity AI8 Discharge Air TemperatureUniversal Inputs UI1 Room Temperature - 8-bit Analog UI2 Hood A SashArea - 8-bit Analog UI3 Hood B Sash Area - 8-bit Analog UI4 ExternalSupply Damper/Humidity Input - 8-bit Analog UI5 External Exhaust FlowInput - 8-bit Analog UI6 Space Emergency Contact - Digital UI7 Hood AEmergency Contact - Digital UI8 Hood B Emergency Contact - DigitalAnalog outputs AO1 Supply Damper Position AO2 Reheat Valve Position AO3Exhaust Damper Position AO4 Hood A Damper Position AO5 Hood B DamperPosition AO6 Auxiliary Reheat Valve Position AO7 Total Exhaust Air FlowAO8 Humidity Cooling Valve Position Digital Outputs DO1 Space EmergencyOutput DO2 Hood A Emergency Output DO3 Hood B Emergency Output DO4 HighLimit Output DO5 Low Limit Output DO6 Digital Output 6 - Unused DO7Digital Output 7 - Unused DO8 Digital Output 8 - Unused

A given module can be networked with up to 32 microprocessors 22 inseries as shown in FIG. 4. The module 82 can be integrated into a STARwhich serves as the controller 18. The STAR is a microprocessor based,multitasking field panel for monitoring and controlling devices whichinclude the communication module 82. More information can be found in adocument entitled “STAR User Manual” American Auto-Matrix part number1E-04-00-0054.

A laboratory is controlled by, for instance, the laboratory controlcircuit 100 as shown in FIG. 6. A discussion of its control sequencefollows. There are preferably four elements of the control sequence.These are temperature, static pressure, humidity and delta flow withrespect to a room under the control of the laboratory controller 100.The four variables that can be manipulated in order to obtain thedesired temperature, static pressure, humidity and delta flow is asupply damper, which controls the amount of air flow into the room; anexhaust damper, which controls the amount of air flow out of the room; acooling valve which dehumidifies the air; and the reheat valve, whichcontrols the amount of heat provided to the flow of air that passesthrough the supply damper into the room.

Specifically, the control sequence with respect to the control oftemperature in the room preferably has two possible procedures that canbe used to introduce additional heat, less heat or the same amount ofheat into the room. The first procedure utilizes a temperature sensor 32which determines the room temperature and provides a correspondingsignal to the measured variable (MV) input of PID No. 2, as shown inFIG. 6. PID No. 2 also receives in its set point (SP) input a roomtemperature set point signal which corresponds to a desired roomtemperature.

The PID No. 2 provides an error signal corresponding to the differencebetween the room temperature and the room temperature set point. Thiserror signal provided by PID No. 2 is then used as the temperature setpoint input which is provided to the SP input of the PID No. 3. The PIDNo. 3 also receives a discharge temperature signal through its MV inputwhich corresponds to the temperature of the air entering the roomthrough the supply damper. PID No. 3 then provides an error signal thatcorresponds to the error between the discharge temperature and thetemperature set point. This error signal from PID No. 3 is then used tocontrol the reheat valve. The output of PID No. 2 can optionally be usedto control an auxiliary reheat valve if one is present in the room.

The control of the reheat valve can be better understood with thefollowing examples. If the error signal, for instance, from PID No. 3indicates that the discharge temperature is not yet hot enough to obtaina desired temperature in the room, then the reheat valve will openfurther to allow additional heat to be supplied to the air flow passinginto the room through the supply damper. If the error signal from PIDNo. 3 is, for instance, too high, then the reheat valve is caused toallow less heat to be provided to the flow of air into the room throughthe supply damper. The error signal from PID No. 3 is also provided to amaximum switch 10 which is involved with the control of the supplydamper. The control of the supply damper will be discussed below.

The second procedure that can be used to control temperature is for thetemperature set point to be manually inputted into the SP input of PIDNo. 3. This is done by toggling a switch 15 disposed between the PID No.2 and PID No. 3 of the lab control diagram such that its 1 input passesthe manual temperature set point signal (as opposed to the 0 input ofthe switch 15 with respect to the first procedure which passes the errorsignal from PID No. 2). By choosing the manual temperature set pointinput, the signal provided by PID No. 2 is eliminated and a fixedtemperature set point is then provided to the PID No. 3. The subsequentoperation of the second procedure for controlling temperature in theroom is the same as the operation of the first procedure described abovefor controlling the temperature starting from PID No. 3.

In order to control the static pressure and the delta flow in the room,a space pressure set point signal is provided to input SP of PID No. 1.The actual space pressure is provided to input MV of PID No. 1. An errorsignal corresponding to the difference in these signals is then producedfrom PID No. 1 and is the delta flow set point signal provided to inputSP of PID No. 4. (Recall that static pressure and delta flow are relatedsince static pressure is constant when delta flow is zero; and staticpressure is changing in the direction of increasing or decreasing deltaflow when delta flow is changing). Thus, PID No. 1 provides the deltaflow set point that corresponds to the difference between the actualstatic pressure in the room and the desired static pressure in the room.If the actual static pressure is the desired static pressure, then thedelta flow set point signal is essentially zero. If the actual staticpressure is different than the desired static pressure, then the deltaflow set point signal corresponds to this difference. The actual deltaflow signal in the room is provided to the MV input of PID No. 4.Optionally, PID No. 1 can be used to directly control the supply andexhaust (static pressure) in a room without utilizing flow sensors byway of switch 117.

PID No. 4 produces an error signal corresponding to the difference inthe delta flow set point signal and the actual delta flow signal. Thiserror signal from PID No. 4 is then provided to the exhaust damperoutput. The exhaust damper is accordingly moved in response to thecommand placed on it from the signal of PID No. 4. The error signal fromPID No. 4 is also provided to the maximum switch 10 to which the errorsignal PID No. 3 is also provided.

The maximum switch 10 allows the greatest of four signals (auxiliaryreheat, reheat, static pressure and external supply) to pass to thesupply damper output. It is the greatest of these four signals whichcontrols the supply damper. This way, for instance, if more heat is tobe provided to the room in order to increase the temperature of theroom, then the supply damper will also increase the flow of heated airinto the room. If, the signal from PID No. 4 is greatest than the signalfrom PID No. 3 due to, for instance, the static pressure, or the deltaflow being increased, then the supply damper will provide a greater flowof air to the room in order to increase the static pressure or the deltaflow.

With respect to the delta flow signal provided to the MV input of PIDNo. 4, the signal is essentially the difference between the supply flowinto the room and the exhaust flow out of the room. The exhaust flow outof the room is determined one of two ways. In the event that a sensordetermines the overall exhaust flow from a room, then this value issubtracted from the supply flow into the room at subtractor 120 to yieldthe delta flow of the room. If the exhaust flow sensor is disposed suchthat it only determines exhaust flow out of the room but does notdetermine the exhaust flow out of the room from a hood A and a hood B,then the total exhaust flow is determined by adding the exhaust flow outof the room plus the addition of the exhaust flow out of hood A and hoodB at summer 125. The aforementioned is reduced to practice in part witha switch 130 toggled to allow the appropriate signal to pass. If thesensor determines the overall flow out of a room, then the switch 130 istoggled to allow a zero input signal to pass through the 1 input of theswitch 130. If the exhaust sensor is disposed such that only the exhaustof the room less the exhaust out of hood A and hood B is sensed, thenthe switch 130 is toggled such that the sum of the exhaust out of hood Aand hood B (accomplished with summer 135) is passed through the input ofthe switch 130 to be added at summer 125 to the exhaust flow from theroom.

Alternatively, the delta flow set point signal can be manually set bytoggling a switch 140 between PID No. 1 and PID No. 4 to only allow amanual flow set point signal to pass through the switches input (asopposed to allowing the delta flow set point signal of PID No. 1 to passthrough the input of the switch 140).

In the event it is desired to manually control the exhaust damper andthe supply damper such that they are fully open or fully shut,respectively, by properly toggling switch 150 and switch 160,respectively, a 100% open or 0% open signal, respectively, is providedto the switches 1 inputs and is passed therethrough to open or close theexhaust and supply dampers, respectively. In this manner, the room canbe quickly depleted of air, if, for instance, a fire or toxic chemicalrelease occurs. If the switch 50 and switch 60 is toggled such that thesignal at their 0 inputs are passed therethrough, then the signals fromPID No. 4 or from the maximum switch 10 is passed to the exhaust orsupply damper, respectively.

Optionally, to maintain a minimum air flow into a room, which providesfor a minimum number of air changes for a given time (per hour) in aroom, the output signal from switch 110 is received by both scaler 162and switch 164 (if it is desired not to maintain a minimum air flow,then switch 164 is set to 0 and the output from switch 110 passesdirectly to switch 160). Scaler 162 receives a minimum and maximum airflow range. Scaler 152 then scales the output signal from switch 110 tobe in an allowable air flow range. The output signal from scaler 162 isthen provided to the SP input of PID No. 8. The MV input of PID No. 8receives a supply flow signal indicating the supply flowing through thesupply damper and outputs a signal to switch 164 when is then providedto the supply damper.

The humidity control is accomplished by PID No. 7 receiving through MVinput the humidity sensed by a humidity sensor in the room. The humidityset point is predetermined and provided to the SP input. The output ofPID No. 7 controls a cooling valve based on the level of humidity in theroom. If the humidity is too high, then the cooling valve is openfurther. This causes the room temperature to drop thus causing thehumidity in the room to drop.

The control sequence with respect to the flow of air through a hood 14is based on, in general, coordinating the sash area of the hood 14 withthe hood exhaust damper opening. When the sash area is increased, thedamper opening is also increased in order to remove the additionalvolume of air that is provided to the hood (because of the increasedsash area) and thus maintain the desired face velocity. When the sasharea is decreased, the damper opening is also decreased in order toprevent the smaller volume of air through the smaller sash area frombeing drawn too quickly through the damper opening. Consequently, thedesired face velocity is again maintained.

The control sequence provides for control of either face velocity forvelocity control, or face velocity multiplied by sash area for flowcontrol (see FIG. 8). If face velocity is chosen as the basis formeasurement, then the one input of switch 200 receives the velocitysignal corresponding to the face velocity of the hood. This facevelocity signal is passed through switch 200 to the MV input of PID No.5 and PID No. 6 (PID No. 5 controls a first hood and PID No. 6 controlsa second hood). Additionally, through switch 210's one input is receiveda face velocity set point signal which is then passed through switch 210to the set point SP input PID No. 5 and PID No. 6. The face velocitysignal received at input SP and an error correction signal is outputtedfrom PID No. 5 and PID No. 6 and provided to switch 220. If switch 220is not toggled to an override position, then the signal outputted fromPID No. 5 and PID No. 6 is then passed directly to the exhaust damper ofthe hood positioning it to be in a desired location. In the event thatthe override mode is toggled on switch 210, then the output signal fromswitch 220 causes the exhaust damper to take a fully opened position andallow the maximum possible exhaust to be obtained.

Alternatively, if the flow control is used as a basis to maintain theexhaust damper, then the 0 input of switch 200 receives the sensed flowthrough the exhaust. This signal is then passed directly through switch200 to the MV input of PID No. 5 and PID No. 6. Switch 210 passesthrough the signal at its 0 input. This signal is the sash area of thehood multiplied by the face velocity set point. This resulting signal isprovided to the SP input of the PID No. 5 and PID No. 6.

The set point signal provided by multiplying the face velocity set pointby the sash area is additionally fed to a PID delay 230 as well as to amultiplier 240. At the multiplier 240, the signal is multiplied by afeedforward gain that provides a course adjustment signal which isreceived by summer 250. Summer 250 adds the course adjustment signalfrom multiplier 240 to a feedforward offset signal. This summed signalis provided to switch 260.

If the velocity mode is toggled, then a 0 output from switch 260 isprovided to the PID No. 5 and PID No. 6. If the sash mode is chosen,then the signal received from summer 250 is passed to the FF input ofPID No. 5 and PID No. 6. The feedforward offset signal is based on theparameters of the system such as the duct configuration and hood size.The ultimate purpose of the feedforward gain and feedforward offsetbeing provided to the set point signal in the sash mode is to allow theexhaust damper to properly compensate for the situation where the sasharea and thus the damper is suddenly changed. The exhaust damper lags intime in terms of how it compensates for this change in sash area. Inorder to eliminate or minimize the offshoot that the exhaust damperexperiences from the sudden change in the sash area, the signal receivedby input FF causes the exhaust damper to move to the desired courseposition. The PID No. 5 and PID No. 6 utilizing the inputs from input MVand input SP then places the exhaust damper in an essentially fineadjustment until it arrives at a desired position.

The set point signal, arrived at by multiplying the face velocity setpoint time to sash area in the sash mode is also provided to PID delay230. The PID delay 230 produces a signal based on the time it takes thedamper to achieve full actuation (provided through the damper delayinput to the PID and based on the maximum flow through the damper whenit is fully opened). This delay signal is provided to switch 270 which,if the sash mode is being utilized, is then provided directly to thehold input of PID No. 5 and PID No. 6. The signal received at the holdinput prevents the PID No. 5 and PID No. 6 from calculating the fineadjustment of the exhaust damper for a period of time determined by thesignal provided at the hold input until the course adjustment has hadtime to reposition the exhaust damper. After the time period has passed,then the fine tuning of the exhaust damper position is allowed tocontinue using the MV input and SP input of PID No. 5 and PID No. 6.

The control sequence provides for control of either face velocity forvelocity control or face velocity multiplied by sash area for flowcontrol.

Accordingly, at least the following features are provided:

1. Fume hood air velocity control for safety of the operator and/orintegrity of the experiment/process.

2. Control of room pressure to maintain safety or to preventcontamination.

3. Control of room temperature and humidity for comfort and for processrequirements.

4. Integration of velocity, pressure and climate control with a directdigital control system.

The sash area of a fume hood is calculated from a formula which permitsseveral methods of measurement which derive their input from a universalinput connected to some type of sensor, usually a multi-turnpotentiometer connected to a drum or pulley which is directly attachedto the sash. The formula takes into account several parameters which canbe programmed by the user to model the specific fume hood and sashsystem.

The formula used for calculating the sash position is shown in thefollowing equation:

 Area=OA+[GA*PI*SR*(UI/255)*[SD+ST+SR*(UI/255)]/12]

Where:

OA Offset Area—minimum sash opening, SQ FT

GA Sash Width—width of sash opening, FT

SR Pot Turns Per 100%—number of turns the post has from endstop toendstop

UI Universal Input—8-bit analog input which measures the pot voltage

PI Pi—3.1416

SD Drum Diameter—diameter of drum or pulley to which the pot isattached, IN

ST Cable Thickness—thickness of the cable used if the pot is attached toa drum which coils the cable, IN

This equation accounts for the added diameter of the drum caused by thecoiled cable. In the case of a system with a pot connected directly to apulley, the ST, cable thickness, attribute would be set to zero and thedrum diameter would simply be SD.

In an alternative preferred embodiment, the present invention pertainsto a system 599 for maintaining a desired air flow through a fume hood14 having a sash 601 and a face 611, as shown in FIGS. 9 and 18. Thesystem 599 comprises a processor, such as a PID 603, for controlling airflow through the fume hood 14 based on at least one gain. The system 599also is comprised of means 500 for automatically tuning the gainconnected to the controlling means. Additionally, the system 599 iscomprised of means for manually inputting information to the tuningmeans 500 so the tuning means 500 automatically provides a desired gainto the PID 603 based on information received from the inputting means.

The tuning means 500 preferably includes means 502 for converting inputinformation to logic values, as shown in FIG. 10, and the manuallyinputting means is a keypad 508 in which crisp input values (other thanlogic values) are entered into the converting means 502. Preferably, theconverting means 502 is comprised of at least one input membershipfunction which defines a number of domains within a range of inputvariables.

The tuning means 502 preferably also includes an inference engine 504for determining an adjustment to the gain based on the logic valuesreceived from the converting means 502. The inference engine 504 isconnected to the converting means 502. The inference engine 504 ispreferably comprised of a set of rules describing the desired systembehavior. The inference engine applies the logic values from theconverting means 502 to the set of rules to produce an adjustment to thegain. The adjustment to the gain is preferably a logic value.

The tuning means 500 preferably also includes means 506 for convertingthe adjustment to the gain to gain output values which are other thanlogic values. The converting the adjustment means 506 preferablycomprises at least one output membership function which defines a numberof domains within the range of the output variable. With the outputmembership function, the logic value received from the inference engine504 is converted to a crisp output value that can be transmitted to thecontrolling means. The converting the adjustment to gain means 506 isconnected to the inference engine 504.

The present invention also pertains to a method for controlling a fumehood 14 with a sash 601 and a face 611. The method comprises the stepsof providing information concerning flow of air through the fume hood 14to a processor, such as a PID 603. Then, there is the step of manuallyinputting information concerning gain into a qualitative tuning system500 connected to the PID 603. Next, there is the step of automaticallyproviding an adjustment to the gain to the PID 603 by the qualitativetuning system 500 based on the information. Next, there is the step ofcontrolling the flow of air through the fume hood 14 with the PID 603based on the gain it receives from the qualitative tuning system 500.The method for qualitatively tuning a processor preferably comprises thesteps of transforming information into an information signal relative togain. Next, there is the step of converting the information signal aboutgain into a logic value signal. Then, there is the step of determiningadjustments to a gain signal based on the logic value signal. Then,there is the step of inputting the adjustment gain signal to theprocessor.

In the operation of the preferred embodiment, a system, such as amicroprocessor 599, controls a single fume hood 14 based on facevelocity, face velocity with sash position-feedforward, or flow reset bysash position. The hood's control parameters are programmable. Themicroprocessor 599 modulates a damper 607 located in the exhaustductwork 609 to maintain a specified face velocity setpoint. A sashposition sensor 619 can be used to anticipate changes in the facevelocity due to sash movement. When control is based on exhaust flow,the sash position and desired velocity are used to calculate acorresponding flow setpoint.

The damper modulation is based on an analog output signal. The outputsignal is selectable for 0-10 VDC, 0-20 mA or 4-20 mA. Non-linearactuators are supported through programmable piece-wise linearizationtables.

Piece-wise linearization tables for the Sierra Instruments 0-200 fpm and0-2000 fpm velocity sensors 621 are provided. Two programmablelinearization tables are available for other types of sensors.

A cascaded flow input is used to pass the exhaust flow from themicroprocessor 599 to another microprocessor 599 for total exhausttracking and control. When connected in this manner, the flow sensorinput value is added to the cascaded flow input value. The result isthen written to the cascaded flow output. A group of the microprocessors599 can be connected to, for instance, a microprocessor 22, to handlethe air flow tracking control.

An emergency override input forces the damper 607 to a programmableemergency position and initiates an alarm condition. One relay output isprovided for alarming. In addition, three non-relay digital outputs canbe programmed for various alarm functions. The apparatus supports PUPalarming (see PUP guidelines identified above), which operates in one oftwo ways. First, the microprocessor 599 will respond to any poll foralarms from a controller 18 or a monitoring device, such as an AmericanAuto-Matrix SAGE™, SOLOFone™, MouseView™, SOLOView™ or SOLOTool™, asshown in FIG. 18. Second, the microprocessor 599 broadcasts its alarmswhen it has the token in a PUP token passing network. The desired alarmfunction is programmable.

The microprocessor 599 contains an integral keypad 503 and display, asshown in FIG. 11, for input monitoring and parameter modification. Theoperator interface is menu driven with password access and Englishlanguage text, although other languages can be used.

The display is a backlit 2 line by 16 column LCD. The operator interfaceprogram provides simple English language menus. The menu structureallows parameters of the microprocessor 599 and other similar devices,such as SOLO products of American Auto-Matrix to be examinedinteractively and modified or adjusted. The microprocessor's 599configuration and setup can also be examined and modified. Allmodification and examination functions may be configured to require oneof several password codes, which are also configurable.

The microprocessor 599 may be used to examine parameters in any otherSOLO based devices connected to the PUP network 16. The microprocessor599 supports PUP version 8 display list conventions and attribute textdescriptions.

A Bicolor LED displays summary status, where green indicates OK, yellowindicates low hood flow and red indicates emergency or extreme limit.The LED may be configured to flash when red or yellow to assist colorblind operators.

The Qualitative Tuning System 500, as shown in FIG. 10, applies a FuzzyLogic approach to assist in tuning the gains of standard PID 603 controlloops. The tuning is achieved by adjusting qualitative parameters whichare inputs to the system 500. These inputs are “fuzzified” and becomeinputs to the Inference Engine 504. Using these inputs, the InferenceEngine 504 evaluates a set of rules which describe the behavior of thesystem. The outputs from the Inference Engine 504 are defuzzified andapplied to the proportional, integral and derivative gains of the PID603 control loop.

The tuning parameters must be converted from crisp input values to fuzzylogic values. The fuzzification process is achieved through inputmembership functions, which define a number of domains within the rangeof the input variable (universe of discourse). For example, the inputparameter and input membership functions for turbulence, response timeand sensitivity, as shown in FIGS. 12, 13 and 14, respectively, werechosen for the qualitative tuning system 500.

The Inference Engine 504 applies the fuzzy logic inputs from thefuzzification process to a set of rules describing the desired systembehavior. These rules produce fuzzy output values. The inference engineevaluates the rules using the Min-Max method of rule evaluation. Allrules are of the form:

IF Antecedent 1 AND Antecedent 2 Then Consequent 1

Antecedent takes the form: Input Variable=Label

Consequent take the form: Output Variable=Label

The qualitative system uses, as an example, the following rules toproduce adjustments (outputs) for the proportional, integral andderivative gains.

Proportional (Kp) Output Rules

IF turbulence is calm AND response time is slow THEN proportional is low

IF turbulence is calm AND response time is moderate THEN proportional islow

IF turbulence is calm AND response time is fast THEN proportional ismedium

IF turbulence is moderate AND response time is slow THEN proportional islow

IF turbulence is moderate AND response time is moderate THENproportional is medium

IF turbulence is moderate AND response time is fast THEN proportional ishigh

IF turbulence is high AND response time is slow THEN proportional ismedium

IF turbulence is high AND response time is moderate THEN proportional ishigh

IF turbulence is high AND response time is fast THEN proportional ishigh

Integral (Ki) Output Rules

IF turbulence is calm AND response time is slow THEN integral is low

IF turbulence is calm AND response time is moderate THEN integral ismedium

IF turbulence is calm AND response time is fast THEN integral is medium

IF turbulence is moderate AND response time is slow THEN integral ismedium

IF turbulence is moderate AND response time is moderate THEN integral ismedium

IF turbulence is moderate AND response time is fast THEN integral ismedium

IF turbulence is high AND response time is slow THEN integral is medium

IF turbulence is high AND response time is moderate THEN integral ismedium

IF turbulence is high AND response time is fast THEN integral is high

Derivative (Kd) Output Rules

IF turbulence is calm AND sensitivity is low THEN derivative is low

IF turbulence is calm AND sensitivity is moderate THEN derivative is low

IF turbulence is calm AND sensitivity is high THEN derivative is medium

IF turbulence is moderate AND sensitivity is low THEN derivative ismedium

IF turbulence is moderate AND sensitivity is moderate THEN derivative ismedium

IF turbulence is moderate AND sensitivity is high THEN derivative ismedium

IF turbulence is high AND sensitivity is low THEN derivative is medium

IF turbulence is high AND sensitivity is moderate THEN derivative ismedium

IF turbulence is high AND sensitivity is high THEN derivative is high

The fuzzy outputs from the inference engine 504 must be converted fromfuzzy logic values to crisp output values. This defuzzification processis achieved through output membership functions, which define a numberof domains within the range of the output variable (universe ofdiscourse). For example, the output parameters and membership functionsfor proportional output, integral output, and derivative output, asshown in FIGS. 15, 16 and 17, respectively, were chosen for thequalitative tuning system 500. Note that the membership functions aresingletons whose values can be calculated using a weighted averagetechnique.

As an example, let the turbulence index chosen by a user be 25 and theresponse time index chosen by a user be 50. Referring to FIG. 12, aturbulence index of 25 translates into a very calm value of 0.25μ and acalm value of 0.25μ. Referring to FIG. 13, a response time index of 50corresponds to a moderate value of 1.0μ. The sensitivity index chosen bythe user is 0, corresponding to a value of 1.0 for very calm, as shownin FIG. 14. Thus, the crisp input values of 25 for the turbulence index,50 for the response time, and 0 for the sensitivity index results in thefuzzy logic values of 0.25μ for very calm and 0.25μ for calm withrespect to turbulence, 1.0μ for moderate with respect to response time,and 1.0μ for very calm with respect to sensitivity.

These fuzzy logic values are then applied to the proportional, integraland derivative output rules of the inference engine using the Min-Maxmethod of rule evaluation. Under the Min-Max method of rule evaluation,the minimum value of the two input variables for a given consequentdefines the output variable. In regard, the proportional output rules,the only rule that yields an output variable other than 0 is “IFturbulence is calm AND response time is moderate THEN proportional islow”. Applying the fuzzy logic inputs to this rule results in an outputvariable for proportional with respect to low equal to 0.25, withrespect to medium equal to 0 and with respect to high equal to 0.

For integral output, the only rule that yields a value other than 0 is“IF turbulence is calm AND response time is moderate THEN integral ismedium”. The value for integral with respect to medium equals 0.25. Thevalue of integral with respect to low equals 0 and the value of integralwith respect to high equals 0. For derivative, the value of low equals0, the value of medium equals 0 and the value of high equals 0.

Fuzzy outputs must be converted from fuzzy logic values to crisp outputvalues for the PID 603. Using the weighted average technique,

crisp output=Σ_(i)(fuzzy out_(i))·(output value_(i))/Σ_(i)(fuzzyout_(i))

The crisp output for proportional is determined with reference to FIG.15. The proportional crisp output equals:

[(0.25×0)_(low)+(0×50)_(medium)+(0×100)_(high)]÷[0.25+0+0]=0.

The integral crisp output value equals:

 [(0.0×0)_(low)+(0.25×50)_(medium)+(0×100)]÷[0+(0.25×50)+0]=12.5÷0.25=50.

The derivative output equals:

[(0×0)_(low)+(0×50)_(medium)+(0×100) _(high)]÷(0+0+0)=0.

Thus, after the defuzzification process, the crisp output forproportional is equal to 0, the crisp output for integral is equal to 5and the crisp output for derivative is equal to 0.

The present invention also pertains to a system for controlling a fumehood 14 having a sash 601 with a face 611, as shown in FIG. 18. Thesystem 600 comprises at least one means for controlling air flow throughthe fume hood based on sash position and air flow face velocity. Thecontrolling means preferably includes a microprocessor 599. This mode offace velocity with sash feedforward control is different than the modesof face velocity control only or flow with sash feedforward controldescribed above. A microprocessor 22, for controlling two fume hoods 14,can also be part of the system 600. The microprocessor 22 is connectedto the PUP network 16. Except for the automatic tunability feature andthe additional control sequence for face velocity with sash feedforwardand the preferred use of only one fume hood 14 connected to it, themicroprocessor 599 in its preferred embodiment is identical to theembodiment described above and shown in FIGS. 3, 4, 6 and 8. The system600 is preferably also comprised of a controller 18 remote from themicroprocessor 599 but in communication with the microprocessor 599. Thesystem 600 preferably includes a telecommunications network connected tothe controller 18 and the microprocessor 599 through which thecontroller 18 and the microprocessor 599 communicate.

The microprocessor 599 preferably includes a processor, such as a PID603, for controlling air flow through the fume hood 14. Additionally,the microprocessor 599 preferably includes means for determining adesired face velocity connected to the processor. Moreover, thecontrolling means preferably includes a sash area sensor 600 incommunication with the microprocessor 599.

The controlling means preferably also includes a damper 607 forcontrolling air flow through the fume hood 14. The damper 607 is incommunication with the microprocessor 599. Moreover, the microprocessor599 preferably includes means for moving the damper to a desiredposition in response to the sash area changing.

Referring to FIG. 9, the means for determining a desired face velocityincludes a face velocity sensor 602 connected to the measured variableinput (MV) of the microprocessor 599 which preferably includes a PID603. The means for determining a desired face velocity also includes aface velocity setpoint 604 that is connected to the setpoint input (SP)of the PID 603. The velocity setpoint 604 is connected to the SP inputport of the PID 603 through switch 610. When switch 610 is toggled toits zero input, the velocity setpoint value is allowed to be transmittedtherethrough.

The system 600 preferably also includes a sash area sensor 606 incommunication with the PID 603 of the controlling means. The means ormechanism for moving the damper 607 to a desired position in response tothe sash area 617 changing is preferably comprised of the following.There is a first multiplier 605 having an A input and a B input whichthen multiplies the A input by the B input. The A input is connected tothe velocity setpoint 604 and the B input is connected to the sash areasensor 606. The multiplication of the velocity setpoint by the sash arearesults in a desired flow through the fume hood. A desired flow valuethat is produced by the first multiplier 605 is connected to a secondmultiplier 607. The second multiplier 607 has an A input which receivesthe desired flow value from the first multiplier 605 and also has a Binput. The B input receives a feedforward gain which is multiplied bythe flow value received by the A input to result in a first feedforwardsignal. The purpose of this feedforward feature is for the damper to beimmediately moved to an expected position to maintain the desired flowthrough the fume hood within seconds of the movement of the sash ratherthan waiting for the flow through the fume hood to change after the sashis moved and then for the damper to be reactive to the actual change offlow. This is the same purpose as it is described above for flow basedfume hood control. One could think analogously of a door being openedand for several seconds to pass before a flow of air comes through thedoor and is felt by the person standing in the doorway that has openedthe door. By moving the damper 607 in advance to an expected positionthat would maintain the desired flow at the new sash position,instabilities of flow are reduced or eliminated. In situations wherehazardous materials are present in the fume hood, this minimizes therisk of the materials potentially escaping into the room through thesash.

The first feedforward signal is provided to a summer 650 which is alsoconnected to a feedforward offset which provides a feedforward offsetsignal thereto. The summer 650 adds the feedforward offset signal to thefirst feedforward signal and produces a second feedforward signal thatis more tailored to the desired feedforward response of the damper.Feedforward offset is used to maintain a minimum feedforward signal atall times. The second feedforward signal is provided to the one input ofthe third switch 660. The third switch 660 also has a zero input whichhas a predefined zero value connected to it. When the third switch 660is toggled to the one input, the second feedforward signal is passedthrough the third switch 660 to the feedforward (FF) input port of thePID 603. The second feedforward signal is then passed immediatelythrough the PID 603 to the output thereof to immediately effect thedamper to move the damper to a desired position in advance of the actualflow through the fume hood changing. If the feedforward feature is notused of the controlling means, then the zero input is toggled on thethird switch 660 and a zero value is provided to the FF port resultingin no feedforward. The third switch 660 has the desired input passedtherethrough based on whether the face velocity with sash feedforwardmode or the flow with sash feedforward mode (as described above) ischosen. This choice is manifested through the or gate 618 which, throughits A input or B input, receives a value indicating whether either ofthese modes have been chosen. If neither of these modes are chosen, thena zero value comes out of the or gate 618 causing a zero value to bepassed through the feedforward indicating there is no feedforwardpresent.

The desired flow from the first multiplier 605 is also provided to PIDdelay 630. The PID delay 630 receives the desired flow through its Ininput. The PID delay 630 also has a Max input that receives a maximumdelta flow signal, and a Time input that receives a damper delay signal.The maximum delta flow is the largest flow change that occurs throughthe hood and is based on the specific hood being controlled. The damperdelay provides the time it takes for the desired damper to go from aminimum to a maximum position. With this information, the PID delay 630can determine how long it will take to move the damper from a givenposition to a new position based on the change in the sash positionwhich appears in the new desired flow signal. The PID delay 630 producesan output signal which equals (the desired flow divided by the maximumdelta flow) times (the damper delay time) resulting in a time value asthe output of the PID delay 630. The time signal produced by the PIDdelay 630 is provided to a fourth switch 670 through its one input. Ifthe feedforward feature is present through the fume hood, the fourthswitch 670 is toggled by a signal from the or gate 618 to allow the timesignal from the PID delay 630 to be provided to a hold input of the PID603. When a value other than zero is received by the hold input of thePID 603, then the PID passes the second feedforward signal directly tothe damper for the period of time provided by the time signal to the PID603 through the hold input. In the event there is some obstruction infront of the sash, but the sash position does not change, for instance,when someone stands in front of the sash, then the face velocity sensor602 will immediately sense the change in the face velocity and provideit to the MV input of the PID 603. The PID 603 through the velocitysetpoint signal it receives in the SP input port then modifies thedamper 607 position to maintain the desired face velocity.

As shown in FIG. 19, a system 700 uses the same control sequences, justdescribed, for controlling two fume hoods 14. FIG. 19 is similar to FIG.7, except FIG. 19 also shows the face velocity with sash feedforwardfeature.

All the control sequences described herein can be used interchangeablyalone or in conjunction with each other through processor 44 and memoryof microprocessor 22 or microprocessor 598, which can be essentially thesame as shown in FIG. 3 except the control sequence in FIG. 9 can bestored and operated in the microprocessor.

In another embodiment, face velocity is determined with means formeasuring pressure at the face of a fume hood, such as an air foil pitot701 which detects pressure across the face of a fume hood, as shown inFIG. 20. See also U.S. Pat. No. 4,928,529, incorporated by reference. Byusing the air foil pitot 701, sash position is not needed to bemeasured. The pressure sensed by the air foil pitot 701 is fed by way ofa pressure signal to means for converting the signal to a face velocitysignal, such as a piecewise linearization 702. The piecewiselinearization 702 converts the pressure into units of velocity. Anexample of a linearization for this purpose is shown in FIG. 21. Anexample of a face velocity linearization curve is found in Table I.

TABLE I Face Velocity Linearization Curve Sensor Input (mA) Sensor Input(counts) Face Velocity (FPM) 4.00 13107 0.0 5.60 18350 49.0 7.20 2359369.0 8.80 28835 84.9 10.40 34078 98.0 12.00 39321 109.6 13.60 44564120.1 15.20 49807 129.7 16.80 55049 138.7 18.40 60292 147.1 20.00 65535155.1

The value from the piecewise linearization 702 is provided through asignal to the measured variable (MU) input of the PID 704. This iscompared to the velocity setpoint that is introduced to the PID 704through the setpoint (SP) input of the PID 704. The PID 704 operates asdescribed above and produces an output control signal to control adamper 710, although the PID 704 does not need to be automaticallytuned. This output control signal from the PID 704 is provided to meansfor characterizing the output control signal, such as a characterization706. The characterization 706 is required because the damper position isnonlinear with respect to the actual measured flow. An example of adamper characterization curve is found in FIG. 22 and Table II.

TABLE II Damper Characterization Curve PID Output (%) Analog Output (%)Analog Output (mA) 0.0 0.0 4.00 10.0 2.0 4.32 20.0 5.0 4.80 30.0 8.05.28 40.0 12.0 5.92 50.0 18.0 6.88 60.0 24.0 7.84 70.0 32.0 9.12 80.050.0 12.00 90.0 70.0 15.20 100.0 100.0 20.00

The characterized output that results from the characterizationcompensates for the nonlinear response of the damper 710. The outputsignal from the characterization 706 is fed to a linear actuator 708that drives the damper 710.

The damper 710 is preferably a precision damper assembly PDA-X ofEastern Instruments Laboratories, Inc. located at 416 Landmark Drive,Wilmington, N.C., as shown in FIG. 23. The damper 710 preferably has aLAP-1 linear actuator with positioner of Eastern Instruments, connectedto it. The PID 710, the piecewise linearization 702 and thecharacterization 706 can all be found on a 68HC11F11 chip of Motorola.

In an air foil bench type fume hood, there is preferably an Air FoilPitot™ AFP installed in the fume hood as is well known in the art with aslack membrane™ transmitter (face velocity) FVR-1A, both of which can beobtained from Eastern Instruments. If the fume hood has no air foil andis either a walk-in or a bench-type fume hood, then a Space PressurePrimary™ 21 SPP from Eastern Instruments can be used. The SlackMembrane™ transmitter FVR-1B also of Eastern Instruments is used withthe Space Pressure Primary™ 21 SPP. Installation of these components ofEastern Instruments is well known in the art.

Although the invention has been described in detail in the foregoingembodiments for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be described by thefollowing claims.

What is claimed is:
 1. A system for controlling a fume hood with a face comprising: a first means for controlling air flow through the fume hood based on air flow face velocity, said controlling means having means for measuring pressure at the face of the fume hood which produces a signal corresponding to the pressure; and a controller, remote from the fume hood controlling means but in communication with the controlling means, said controller being capable of receiving a signal from the controlling means and sending a control signal to the controlling means.
 2. A system as described in claim 1 wherein the controlling means includes a microprocessor in communication with the controller and the measuring means.
 3. A system as described in claim 2 wherein the microprocessor includes means for converting the signal from the measuring means into a face velocity signal.
 4. A system as described in claim 3 wherein the measuring means includes a pitot tube assembly or space pressure sensor assembly in communication with the converting means of the microprocessor.
 5. A system as described in claim 4 wherein the controlling means includes a damper for controlling air flow through the fume hood in communication with the microprocessor, and a linear actuator connected to the damper for moving the damper to a desired position in response to change in pressure at the face of the fume hood.
 6. A system as described in claim 5 wherein the microprocessor produces an output control signal and the controlling means includes means for characterizing the damper control signal and then providing it to the linear actuator.
 7. A system as described in claim 1 wherein the controlling means includes a damper for controlling air flow through the fume hood, said damper being in communication with a microprocessor, and an actuator connected to the damper for proportionally moving the damper to a desired position in response to a change in air flow face velocity.
 8. The system of claim 1, wherein the control signal corresponds to a face velocity set point for the fume hood.
 9. The system of claim 1, wherein the control signal corresponds to a damper position for the fume hood.
 10. The system of claim 1, further including a second means for controlling air flow through a second fume hood.
 11. The system of claim 10, wherein the controller is capable of receiving a signal from the second controlling means and sending a control signal to the second controlling means.
 12. The system of claim 11, wherein the controller, the first controlling means, and the second controlling means are interconnected.
 13. The system of claim 12, wherein the interconnection operates in the half-duplex mode.
 14. A method for controlling a fume hood comprising: receiving a control signal from a remote fume hood controller at a local fume hood controller located at the fume hood; receiving a signal from an air pressure sensor; and sending a signal to an exhaust damper based on the received control signal from the remote fume hood controller and the received signal from the air pressure sensor.
 15. The method of claim 4, wherein the received control signal from the remote fume hood controller corresponds to a set point.
 16. The method of claim 4, wherein the received control signal is received over an RS485 network.
 17. The method of claim 4, further including the step of using a microprocessor to generate a signal proportional to a position for the exhaust damper based upon the received control signal from the remote fume hood and the received signal from the air pressure sensor. 